Ink-jet recording is known as a superior recording method that is not much affected by the types of recorded materials. Recording apparatuses, recording methods, recording materials and the like based upon ink-jet recording have been actively studied and developed. At present, dye inks containing dyes as colorants are mainly used for ink-jet recording inks.
Meanwhile, recording media each including an ink absorption layer having voids for absorbing ink in a support base, and further including a porous glossy layer in accordance with the necessity are mainly used for ink-jet recording media. Paper of this type is designed placing importance on matching with dye inks which are currently popular, and already widely used as ink-jet paper, particularly as glossy paper. When such paper is used, very high-definition images superior in glossiness can be obtained; however, since materials therefor are very expensive and also production processes are complicated, the production cost thereof is very high in comparison with the production cost of gloss coated paper for use in commercial printing applications. For that reason, applications of such paper tend to be limited to the case where high-quality output is needed, which is exemplified by photographic output, and in reality it is difficult to use such paper in the field of commercial printing where mass production is required at low prices, which is exemplified by handbills, catalogues and pamphlets.
Also, as to the design concept for long-term storage stability of images produced, since dyes themselves are not highly resistant to ultraviolet rays or ozone, such a method is mainly employed that dyes are made to soak into ink receiving layers of the ink-jet recording media as deeply as possible so as to minimize the effects of the air and ultraviolet rays, and also the dyes are protected with an antioxidant or stabilizer that has been previously added into the ink receiving layers. For that reason, it is necessary to use large amounts of inks in which the concentration of colorants is lowered, and thus printing costs increase. Also, when images or the like are printed onto plain paper, there is a problem that the image quality lowers because of bleeding, for instance.
In order to solve such problems, pigment inks using pigments as colorants have been proposed for ink-jet recording inks in recent years. The pigment inks are superior to the dye inks in image density, water resistance and weatherability and make it possible to obtain images with less bleeding at the time of printing onto plain paper; however, at the time of printing onto recording media having coating layers, there are such problems that sufficient color-developing ability and glossiness cannot be obtained, and image-fixing ability degrades because it takes a longer time for the pigment inks to dry after printing.
Also in the case where black letters/characters, black lines and black images, generally used in many applications, are printed only using black pigment inks, the pigments flocculate and adhere in the vicinity of the surfaces of recording media; therefore, the surface state of recording media greatly affects the image density, and there are such troubles caused that the image density decreases and the image quality becomes poor especially at the time of printing onto recording media having coating layers. This tendency is conspicuous when the pigment concentration is reduced instead of increasing the nonvolatile content or sparingly volatile content (amount of wetting agents, excluding penetrants) in water-soluble organic solvents that are components of the black pigment inks as an attempt to improve ejection reliability and lower the costs of the black pigment inks, which are most frequently used.
In order to solve such problems, Patent Literature 1 to Patent Literature 3 propose ink-jet recording methods, each of which is a method wherein a black ink containing a black pigment, and color inks containing dyes of cyan, magenta and yellow are prepared, and an image is recorded onto a recording medium. However, these proposals in related art are not characterized by using pigment inks for all colors; also, dyes themselves are not highly resistant to ultraviolet rays or ozone, so that the use of dyes for color inks makes it necessary, for the same reason as described above, to use large amounts of inks in which the concentration of colorants is lowered, and thus there is such a drawback that printing costs increase.
Patent Literature 4 discloses that it is possible to obtain printed text having a high density and superior light resistance by printing a black image of composite black with the use of a combination of a self-dispersible black pigment ink and resin-coated color pigment inks. However, the pigment concentration of the black ink described in Examples is not lower than 4% by mass, and use of a black ink having a pigment concentration of 3% by mass or less as in recording inks of the present invention is not stated.
Patent Literature 5 proposes use of a self-dispersible pigment or a colorant coated with a polymer as an ink-jet recording method for obtaining sufficient image density, not affected by the types of media such as plain paper and glossy media. According to this proposal, a black image of composite black can be obtained mainly by means of inks which have colors other than black and contain colorants coated with a polymer, when the medium used is a glossy medium; however, the ink-jet recording method is different from the method for obtaining a recorded matter superior in image density and glossiness by printing an image of composite black with the use of inks including a black ink, at which the present invention is aimed.
Patent Literature 6 discloses that glossiness and image clarity on ink-jet paper such as glossy paper or photo paper can be improved by employing a recording solution containing a water dispersion of water-insoluble vinyl polymer particles. In the method for obtaining a black image of composite black by using this recording solution, the black image is printed using an ink set provided with a magenta ink, a yellow ink and a cyan ink; therefore, unlike the present invention, it is not that a recorded matter superior in image density and glossiness is obtained by printing an image of composite black with the use of inks including a black ink. Moreover, recording media in Patent Literature 6 are ink-jet paper such as glossy paper or photo paper, which is different from inexpensively available gloss coated paper in the present invention for use in commercial printing applications.
Patent Literature 7 proposes an ink-jet recording apparatus for printing high-quality images, wherein a printing head is provided with two black ink nozzles for ejecting two different black inks, edge areas of a black area that is a recording target are recorded using a black pigment ink serving as a second black ink, and an inner area sandwiched between the edge areas is recorded, alternately using the second black ink and a black dye ink serving as a first black ink which is higher in penetrating speed than the second black ink. However, the ink-jet recording method in this proposal is different from the method for obtaining a black image of composite black formed using a combination of a black ink and color inks as in recording inks of the present invention; also, there might be a concern that due to the use of dye ink, images produced in the proposal would be poorer in image storage stability than images formed only using pigment inks.
As just described, it is, in reality, still difficult to obtain a high image quality recorded matter that is excellent in image density, glossiness and reliability on recording media and that is close in quality to recorded matters produced by commercial printing such as web offset printing.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2000-198227    [Patent Literature 2] JP-A No. 2006-168372    [Patent Literature 3] JP-A No. 2006-182026 [Patent Literature 4] International Publication No. WO2002/026898    [Patent Literature 5] JP-A No. 2002-327138    [Patent Literature 6] JP-A No. 2006-316246    [Patent Literature 7] JP-A No. 2002-113850